Stators having a permanent magnet system comprising a plurality of individual permanent magnets are known in the art. In a known stator (U.S. Pat. No. 3,237,036) a plurality of permanent magnets are united into a star-shaped assembly held together by strap rings and supported on a radially inner hollow stem. The permanent magnets are made in a substantially rectangular parallelepiped form and are magnetized in a circumferential direction. Wedge-shaped pole pieces are arranged between adjacent ones of the permanent magnets. The unit formed by the permanent magnets and pole pieces is surrounded by a flux return ring, leaving a cylindrical air gap. No provision is made to allow an adjustment of the pole pieces to close tolerances of the air gap. Therefore, the permanent magnets as well as the pole pieces must be machined to close tolerances in order to get a close toleranced air gap.
In order to reduce the cost of manufacture, a number of permanent magnets have been fixed around an inner magnetic conductive piece. Such a construction, however, requires a precise treatment of the surface of the permanent magnets and of the inner magnetic conductive piece, and thus the reduction of costs of manufacture is very poor.